Differential gear systems for vehicles are well known in the art. A differential gear system allows a vehicle to travel in a curve without dragging one wheel of a pair of powered wheels. When a vehicle travels in a curve, the inner and outer wheels of the vehicle rotate at different speeds because the wheels on the outside of the curve must travel a greater distance than the wheels on the inside of the curve. Where either the front or rear wheels are not powered but rather spin freely, the outer wheel simply turns faster than the inner wheel. A differential rotating speed, however, is not possible if both wheels of a pair of powered wheels are attached to a single, continuous axle. For that reason, the drive shaft of a vehicle typically does not transfer power from the vehicle's engine directly to a continuous axle that transfers power to a pair of wheels. Instead, the drive shaft is coupled to a differential gear mechanism that drives separate wheel shafts (also called “half shafts” or “universal shafts”). Each wheel shaft is coupled to and drives a wheel. The differential gear system allows each wheel of a pair of powered wheels to rotate at a different speed.
FIG. 1 (prior art) shows a conventional differential gear system 10 from a top perspective in a rear-wheel-drive vehicle. The forward driving direction of the vehicle is oriented towards the top of FIG. 1 such that a right rear wheel 11 is shown to the right. Differential gear system 10 includes a differential casing 12 in which two bevel gears 13–14 and two beveled pinions 15–16 are rotatably mounted. The teeth of each bevel gear mesh with the teeth of the two pinions 15–16. In addition, the teeth of each pinion mesh with the teeth of the two bevel gears 13–14. Thus, bevel gears 13–14 and pinions 15–16 rotate in unison. Bevel gear 14 is attached to a right rear wheel shaft 17, which in turn is attached to right rear wheel 11. Bevel gear 13 is attached to a rear left wheel shaft 18. A larger bevel gear wheel, called a crown wheel 19, is attached to the outside of differential casing 12, for example, by bolts 20 that pass through both differential casing 12 and crown wheel 19. Differential casing 12 is mounted on the vehicle such that the entire casing can rotate about the axis of right and left wheel shafts 17–18. For example, differential casing 12 may be enclosed within a housing (not shown) that does not rotate.
Crown wheel 19 has beveled teeth 21 that mesh with teeth of a bevel pinion 22 attached to the end of the vehicle's drive shaft 23. Power from the vehicle's engine is transferred through a gearbox or transmission system to drive shaft 23 and then to differential gear system 10. A rotation of bevel pinion 22 on drive shaft 23 causes crown wheel 19 and the entire differential casing 12 to rotate. When differential casing 12 rotates, pinions 15–16 are carried by differential casing 12 and revolve about the axis of wheel shafts 17–18. When the vehicle is traveling straight ahead, there is no relative motion between bevel gears 13–14 and pinions 15–16. Because wheel shafts 17–18 are attached to bevel gears 13–14, wheel shafts 17–18 rotate at the same rotational speed as differential casing 12, and right wheel shaft 17 rotates at the same speed as left wheel shaft 18.
When the vehicle is traveling in a left curve, for example, differential gear system 10 allows right wheel shaft 17 to rotate faster than left wheel shaft 18 such that right rear wheel 11 on the outside of the curve can travel a greater distance than the left rear wheel on the inside of the curve. Because bevel gears 13–14 and pinions 15–16 rotate in unison, bevel gear 13 rotates in an equal and opposite direction from bevel gear 14. Thus, right rear wheel 11 rotates faster than differential casing 12 by the same difference in rotational speed as the left rear wheel rotates slower than differential casing 12.
Various modifications to the conventional differential gear system have been proposed. Efforts have been made to reduce the size and weight of the differential gear system and its components. Reducing the weight improves the overall fuel efficiency of the vehicle carrying the differential gear. Reducing the size of the differential gear system allows the space savings to be used for other purposes, such as to expand the gas tank or trunk of the vehicle.
One modification has been to construct a differential gear using worm gears as described in U.S. Pat. No. 6,582,338. The torque capacity associated with worm gears, however, can be limited. Another modification employs multiple planetary pinions with helical teeth surrounding sun gears. U.S. Pat. No. 6,634,979 describes a differential gear system in which a sun gear is attached to each wheel shaft. Each sun gear has outer helical teeth that mesh with helical teeth on six planetary pinions.
These modifications do not necessarily reduce the size and weight of the differential gear system. In addition, these modifications do not necessarily reduce the number of components within the differential gear system. Reducing the size and number of components can decrease the cost of manufacturing the differential gear system. Moreover, as the number and complexity of the components decreases, the durability of the system tends to increase and maintenance costs are reduced.
Modifying the conventional differential gear system containing beveled gears by employing worm gears or gears with helical teeth does not necessarily achieve the desired advantages of reduced size, weight and cost and can even result in decreased torque capacity. A differential gear system is sought that does not employ beveled gears, worm gears or gears with helical teeth.